The present invention generally relates to a gas diffusion element a method of manufacturing the same, and a device using the same.
More particularly, it relates to a gas diffusion element which can be used in gas diffusion electrodes for proton exchange membranes for fuel cells on membrane electrode assembly base and/or as a catalyst carrier of hydrogen generators/hydrogen convertors/hydrogen reformers, etc.
A complex structure of a gas diffusion electrode has not been produced in an integrated form and the art has relied on manual preparation of separate individual layers using expensive matrix materials such as polytetrafluoroethylene doped for hydrophobicity and expanded for porosity, together with precious metals for catalysis, and carbon for conductivity and a metal grid current collector. These layers are then bonded together to form the complex structure. Such prior art electrodes are expensive to produce, have failures at the layer junctions and do not provide good performance or utilization of the materials.
Production methods using hand-lay-up electrode rolling and catalyst addition or multiple hot press sinter stage production cannot become cost effective because of labor intensity and because of batch to batch variability. It is, therefore, essential that continuous production methods be developed which overcame the substantial production challenges, wherein the material costs must be low; the layers within the electrode structure must be integrated, and that the hydrophobic, catalyst and current collector sub-layers need to be optimized and not delaminate or degrade during use.
U.S. Pat. No. 4,058,482, issued Nov. 15, 1977, discloses a sheet material principally comprised of a polymer such as PTFE and a pore-forming material, wherein the sheet is formed of co-agglomerates of the polymer and the pore former. This patent teaches mixing polymer particles with positively charged particles of a pore former, e.g., zinc oxide, to form co-agglomerates thereof followed by mixing same with a catalyst suspension so as to form co-agglomerates of catalyst and polymer-pore-former agglomerates followed by pressing, drying, and sintering these co-agglomerates. Subsequent to this sintering, the pore former can be leached out of the electrodes. Thus, this teaching requires use of pore former agglomerates, which require subsequent leaching and not an integrated electrode structure.
U.S. Pat. No. 4,150,076, issued Apr. 17, 1979, is directed to a process for forming the sheet of aforesaid U.S. Pat. No. 4,058,482, and distributing same as a layer on a suitable electrode support plate, for example a carbon paper, to form a fuel cell electrode by a process which includes pressing, drying, sintering, and leaching. It discloses use of an electrode support plate such as carbon paper in addition to having the same limitations as U.S. Pat. No. 4,058,482.
U.S. Pat. No. 4,170,540 to Lazarz et al, issued Oct. 9, 1979, discloses a microporous membrane material suitable for electrolytic cell utilization, e.g., as a chlor-alkali cell separator, and formed by blending particulate polytetrafluoroethylene, a dry pore-forming particulate material, and an organic lubricant. These three materials are milled and formed into a sheet which is rolled to the desired thickness, sintered, and subjected to leaching of the pore-forming material. The electrode contains no active carbon and does not function as a catalytic or active layer in an electrode. The microporous layer will not be conductive and, hence, would not permit easy electron transfer through the layer.
U.S. Pat. No. 4,152,489, issued May 1, 1979, describes air electrode structures consisting of a metal grid active material such as acetylene black carbon and catalyst with porous sintered polytetrafluoroethylene require considerable force to combine these materials into a single structure. The force collapses the pores and reduces pore volume and hence reduces active catalyst loading area. It described “a multi-layer hydrophilic section, consisting of at least two substantially uncompacted laminated layers of plaques containing loaded, catalytically active battery material; each loaded plaque consisting of a 75% porous to 95% porous metal current collector to put more catalyst into the pore structure by preparing the metal grid with catalyst before pressing.
U.S. Pat. No. 4,354,958, issued Oct. 19, 1982, described an improved fibrillated matrix-type active layer having improved strength and durability so as to avoid mechanical failure during use. The process for forming this structure is complex. Components are shear-blended, dried, chopped into fine form and then rolled. Carbon black particles are combined with an aqueous dispersion of polytetrafluoroethylene (PTFE) and are then dried before combining with active carbon and finally forming the layer.
U.S. Pat. No. 4,459,197, issued Jul. 10, 1984, describes a laminated electrode with an active layer, a current distributor and a hydrophobic layer wherein the active layer has a carbon black with a surface area of 20 to 1500 square meters per gram. The electrodes described do not require fillers to make the layers porous. Discussion is made of prior art with reference to the electrodes having poor mechanical strength. Use of aforesaid U.S. Pat. No. 4,354,958 as an active layer is described.
U.S. Pat. No. 4,500,647, issued Feb. 19, 1985, describes the preparation of a 3 layer laminated matrix which uses carbon particles in a PTFE layer prepared from an aqueous dispersion, drying, then blending the material with active carbon before producing an active layer which is laminated to a collector and a hydrophobic wetproof layer. The 3 layers are not integrated, PTFE is required which is expensive, the development of pore structure is not included and the active layer is laminated to a current distributor.
U.S. Pat. No. 4,514,474, issued Apr. 30, 1985, describes an air electrode having an active layer with a collector grid on one side and a porous nonconductive separator on the other. The nonconductive separator is described as porous polypropylene. The grid and separator are permanently bonded to the active layer and the polypropylene was preselected with porosity. The layers are not integrated, nor can the porosity in the nonconductive separator be developed. The non-conductive porous separator has a disadvantage by not providing a structure for oxygen reduction and electron capture.
U.S. Pat. No. 4,518,705, issued May 21, 1985, describes the preparation of a 3 layer laminated electrode using a technique similar to U.S. Pat. No. 4,500,647, and has the same limitations.
U.S. Pat. No. 4,568,442, issued Feb. 4, 1986, describes a 2 layer composite gas diffusion electrode with an active layer and a backing layer. The active layer is porous, homogeneous and hydrophilic containing hydrophobic coated particulate matter with a catalyst. The active layer is prepared in two parts with the first having a liquid dispersion medium and the second being a prepared dried precipitate which is then shear blended with the first part before calendaring and sintering. It is only a 2-layer structure requiring multistep preparation of materials for the active layer. The active layer is a homogeneous type layer requiring precious metal catalysts and which is bonded to the backing layer. The drying of one part of the active layer before blending with the thermoplastic halocarbon binder and other components such as catalysts is time and energy costly. There is no direction on how to provide an integratable structure and make the bond between the two materials strong.
U.S. Pat. No. 4,602,426, issued Jul. 29, 1986, describes a method of producing a gas diffusion electrode using a series of dry mixtures comprising a hydrophobic agent, a variety of catalyst concentrations and a pore forming agent which are layered on an electrically conductive current collector and then pressed to form an electrode and then leached to produce pores. It does not produce an integrated structure. The dry mixtures are difficult to handle and homogenize, pressing does not produce an integratable structure, the collector is not integrated into the structure but is a separate layer which can be anticipated to have bonding failure at the interface because of different coefficients of thermal expansion.
U.S. Pat. No. 4,636,274, Jan. 13, 1987, describes the formation of a gas depolarized electrode with a carbonized ribbed porous structure, a press molded fluorocarbon layer and an oxygen catalyzing layer. The layer is claimed to be leakage resistant to 3 psi internal pressure. This reference does not teach integration of parts or development of the carbon catalyst active layer in the current invention.
U.S. Pat. No. 4,696,872, issued Sep. 29, 1987, describes the use of a 3 step process to produce a catalytic layer for a fuel cell which involves separately combining two different carbons with binders and then blending these binders and then forming a single layer. It makes use of the different properties of carbon particles but does not teach how to optimize the performance of the two types of carbon but uses a blending of the polymer-coated carbons. It does not describe how to generate porosity in the active layer to allow efficient oxygen ingress or how to produce an air electrode structure, only an active layer.
U.S. Pat. No. 4,737,257, issued Apr. 12, 1988, describes an electrode having a conductive carbon fiber current collector but not how to produce an integrated complex electrode structure.
U.S. Pat. No. 4,877,694, issued Oct. 31, 1989, describes an air electrode having extended performance at high current density having gas porous hydrophobic layer, a hydrophilic halogenated polymer binder with catalyzed carbon particles and particulate bound with hydrophobic polymer which can be pressed onto a carbon paper and supporting metal mesh. It does not describe an integrated structure. The current collector is a pressed layer subject to resistance loses and contact failures with the air electrode. The device uses expensive halogenated polymer binding.
U.S. Pat. No. 4,885,217, issued Dec. 5, 1989, describes an air electrode comprising a sheet-like laminate including (a) first and second layers having opposed major surfaces respectively exposed for contact with a liquid electrolyte and with air, said layers also having facing major surfaces, and said second layer being permeable to air but not to said liquid electrolyte; and (b) current-collecting means in contact with said first layer and connectable to external electrical circuitry; wherein the improvement comprises (c) said first layer comprising a nonwoven fibrous web impregnated with a mixture of carbon particles and a nonfibrous polymeric substance for holding the carbon particles in the web; and (d) said facing major surfaces of said first and second layers being bonded together by heat seal coating material distributed on said facing major surfaces in such manner as to provide an array or network of areas free of coating material extending substantially uniformly thereover. This air-electrode requires a heat sealing coating material which causes blockage of the hydrophobic and hydrophilic layers and ultimate failure of the bond during use.
U.S. Pat. No. 4,906,535, issued Mar. 6, 1990, describes the use of carbon fibers and carbon particles in an air electrode and is a continuation of U.S. Pat. No. 4,885,217.
U.S. Pat. No. 5,312,701, issued May 17, 1994, describes a process for preparing a single pass gas diffusion electrode by a wet filtration technique to collect catalyzed carbon black, hydrophilic fluorinated polymer and a particulate fluorinated polymer from a liquid. A second layer with different properties can be filtered on top of the first. Through the wet filtering there is some mixing of the layers. Sintering is then used to create a single structure. A conductive metal mesh collector can be incorporated in the sintering process which is carried out above 270° C. The intermixing of the wet filtration layers and the single pass production are stated to be advantages in that delaminating is reduced because of the intermixing zone and only a single process step is required to create the two layers. However, there is a need for an improved air-electrode having enhanced longer life dimensional stability made from low cost materials by low cost production without the use of precious metals or expensive fluorinated polymers.